One of the major objectives of an artificial knee joint is to restore the mechanics of the anatomic knee. However the anatomic knee is complex, consisting of structures such as the articular surfaces, ligaments, and muscles, which cannot all be replicated by artificial parts at this time. This problem is mitigated by the fact that arthritis involves destruction of the condylar bearing surfaces and the menisci, while the remaining parts of the knee are largely intact and can still function. Hence, if an artificial knee joint provides a replacement for the bearing surfaces using metal and plastic materials, reasonably normal joint function can theoretically be achieved. This is the fundamental principal behind the large majority of the resurfacing types of knee joint that have been designed since about 1970.
There have been a number of design features introduced to optimize the configurations of artificial knees. Dishing of the tibial bearing surfaces has increased the inherent stability of the artificial knee and has also reduced the contact stresses on the plastic, to increase durability. This dishing has been considered especially important in many designs, because of the requirement to resect the anterior cruciate ligament. Even when it has been present, the difficulty of the surgical technique in retaining the anterior cruciate ligament has led to a preference for resection. Surgical considerations have also led to the further step of resecting the posterior cruciate. When both the anterior and posterior cruciate ligaments are resected, the dishing of the tibial surfaces becomes even more important to provide anterior-posterior, as well as rotational, stability to the knee. When there is only a compressive force across the knee, and no shear force, the femur will locate at a “dwell point” or “bottom-of-the-dish” in the plastic tibial surface. This dwell is constant throughout the range of flexion in a standard total knee design.
A feature of normal knee motions that was seen as an advantage was posterior displacement of the femur on the tibia as flexion proceeded. This was thought to increase the lever arm of the quadriceps and to allow for a higher range of flexion. In configurations which retained the posterior cruciate ligament, this motion was achieved so long as the placement of the components allowed for the natural tightening of the posterior cruciate with flexion. But in configurations where both cruciates were resected, some other mechanism was required to obtain the posterior displacement.
The most widely used mechanism was an intercondylar cam, where a plastic post in the center of the tibial component, projected upwards into a metal housing between the metallic medial and lateral femoral condyles. In one of the first artificial knees using an intercondylar cam (Walker et al, U.S. Pat. No. 4,209,861), the housing articulated against the post throughout the entire flexion range, producing continuous rollback. In subsequent designs, the cam came into contact in the mid-range of flexion. This type of artificial knee configuration has been termed posteriorly stabilized, or PS, knees.
The large majority of the knees implanted today are of the posterior cruciate retaining (CR) or PS types. These have functioned well, but a number of disadvantages remain. Due to variations in surgical techniques, it has been difficult to obtain an optimal tightness with the CR of the posterior cruciate throughout flexion, resulting in variable rollback patterns. For the PS, considerably extra bone needs to be resected from the center of the femur to accommodate the intercondylar housing of the femoral component, there is often long-term damage to the plastic post, and there is a tendency for overconstraint in rotation due to the dishing of the tibial bearing surfaces.
A disadvantage common to both CR and PS types, although more pronounced with CR designs due to their shallower tibial bearing surfaces, is a paradoxical motion in which the femur slides forwards on the tibial surface during flexion, rather than backwards which is the required motion. Also, uneven or jerky motion occurs in many cases. These abnormalities occur because in flexion, the smaller distal-posterior sagittal radius of the femoral component comes into contact with the tibial bearing surface resulting in less constraint to anterior sliding of the femur. This dilemma was partially addressed for mobile bearing condylar knees, when femoral-tibial conformity was a particular goal, as in U.S. Pat. Nos. 5,906,643 and 6,264,679B1 to Walker wherein a solution was disclosed in which notches at the sides of the lateral and medial condyles, continuations of the distal-posterior radius, articulated with matching surfaces on the plastic tibial component. This arrangement was claimed to extend the degrees of flexion over which the condyles were in close contact. One of the main rationales for this arrangement was to provide a single anterior contact in early flexion and a progressively posterior contact in late flexion.
Another feature of anatomical knee motion, as well as a general posterior translation of the femur on the tibia, is rotation of the tibia about its long axis. There are two ways in which this rotation can occur. Firstly, as the knee flexes, there is usually a continuous internal rotation of the tibia. The axis has been found to locate on the medial side of the knee and hence the lateral femoral condyle displaces posteriorly, but the medial femoral condyle displaces only a small amount in comparison. While artificial knees have included configurations that accommodate medial pivotal rotation, these configurations lack an integral mechanism that promotes the anatomical motions of rollback in flexion, medial pivotal rotation and roll forward in extension.
The second type of rotation, termed laxity, is that at any angle of flexion, the tibia can be rotated internally and externally with respect to the neutral position. The amount of rotation towards extension is about 10 degrees on either side of neutral, but this increases to about 20 degrees on either side of neutral by about 30 degrees flexion. In high flexion the laxity apparently reduces again, but there is little quantitative data available. In condylar replacement artificial knees, laxity occurs due to the partial conformity between the femoral and tibial bearing surfaces. The shallower are the tibial surfaces, the greater the laxity. Towards extension, there is closer conformity and less laxity, compared with the laxity in flexion. In a general way, this laxity reproduces the situation in an anatomic knee joint. While partial conformity in an artificial knee is desirable in providing laxity, the contact point tends to be located at the ‘bottom of the dish’ such that it does not displace anteriorly and posteriorly in extension and flexion, nor does it rotate internally with flexion.
An ideal artificial knee configuration would be one where the neutral path of motion, and the laxity about that neutral path, was similar to that of the anatomic knee. In this way, the motion would be compatible with the surrounding soft tissue envelope and the musculature of the knee. In our configuration, the femoral and tibial bearing surfaces are constructed to replace the mechanical function of the anatomic cartilage surfaces, the menisci, and the cruciate ligaments. A surface design with smooth transitions of curvatures has the advantages that large contact areas can be maintained, the motion will be smooth, and there is no possibility of damage to cams or other projections. In fact, in extreme motion conditions or if the artificial joint has been installed so that it is too loose in flexion, actual dislocation over a cam post can occur. For a surface configuration with no cam or projections, this could not occur.
The subject of this invention is a surface design of an artificial knee with continuous curvatures whereby posterior displacement occurs automatically with flexion, with the effect that the dwell point moves progressively posterior with flexion, as in the anatomic knee. Conversely, as the knee is extended from a flexed position, the dwell point moves anteriorly. To more closely replicate the neutral path of motion of the anatomic knee. The surfaces can be designed or configured so that the neutral path in flexion involves internal rotation of the tibia, and conversely as the knee is extended again. Finally the surfaces can also be made to minimize paradoxical motion in early flexion. In all these schemes, the surfaces are such that rotational laxity is possible about the neutral path as in the anatomic knee.